test-writer

• After writing a new concept page
• When adding new code examples to existing pages
• When updating existing code examples
• To verify documentation accuracy through automated tests
• Before publishing to ensure all examples work correctly

Follow these four phases to create comprehensive tests for a concept page.

Phase 1: Code Example Extraction

Scan the concept page for all code examples and categorize them:

| Category | Characteristics | Action |

|----------|-----------------|--------|

| Testable | Has console.log with output comments, returns values | Write tests |

| DOM-specific | Uses document, window, DOM APIs, event handlers | Write DOM tests (separate file) |

| Error examples | Intentionally throws errors, demonstrates failures | Write tests with toThrow |

| Conceptual | ASCII diagrams, pseudo-code, incomplete snippets | Skip (document why) |

| Browser-only | Uses browser APIs not available in jsdom | Skip or mock |

Phase 2: Determine Test File Structure

```

tests/

├── fundamentals/ # Concepts 1-6

├── functions-execution/ # Concepts 7-8

├── web-platform/ # Concepts 9-10

├── object-oriented/ # Concepts 11-15

├── functional-programming/ # Concepts 16-19

├── async-javascript/ # Concepts 20-22

├── advanced-topics/ # Concepts 23-31

└── beyond/ # Extended concepts

└── {subcategory}/

```

File naming:

• Standard tests: {concept-name}.test.js
• DOM tests: {concept-name}.dom.test.js

Phase 3: Convert Examples to Tests

For each testable code example:

1. Identify the expected output (from console.log comments or documented behavior)
2. Convert to expect assertions
3. Add source line reference in comments
4. Group related tests in describe blocks matching documentation sections

Phase 4: Handle Special Cases

| Case | Solution |

|------|----------|

| Browser-only APIs | Use jsdom environment or skip with note |

| Timing-dependent code | Use vi.useFakeTimers() or test the logic, not timing |

| Side effects | Capture output or test mutations |

| Intentional errors | Use expect(() => {...}).toThrow() |

| Async code | Use async/await with proper assertions |

---

Import Pattern

```javascript

import { describe, it, expect } from 'vitest'

```

For DOM tests or tests needing mocks:

```javascript

import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'

```

DOM Test File Header

```javascript

/**

* @vitest-environment jsdom

*/

import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'

```

Describe Block Organization

Match the structure of the documentation:

```javascript

describe('Concept Name', () => {

describe('Section from Documentation', () => {

describe('Subsection if needed', () => {

it('should [specific behavior]', () => {

// Test

})

})

})

})

```

Test Naming Convention

• Start with "should"
• Be descriptive and specific
• Match the documented behavior

```javascript

// Good

it('should return "object" for typeof null', () => {})

it('should throw TypeError when accessing property of undefined', () => {})

it('should resolve promises in order they were created', () => {})

// Bad

it('test typeof', () => {})

it('works correctly', () => {})

it('null test', () => {})

```

Source Line References

Always reference the documentation source:

```javascript

// ============================================================

// SECTION NAME FROM DOCUMENTATION

// From {concept}.mdx lines XX-YY

// ============================================================

describe('Section Name', () => {

// From lines 45-52: Basic typeof examples

it('should return correct type strings', () => {

// Test

})

})

```

---

Pattern 1: Basic Value Assertion

Documentation:

```javascript

console.log(typeof "hello") // "string"

console.log(typeof 42) // "number"

```

Test:

```javascript

// From lines XX-YY: typeof examples

it('should return correct type for primitives', () => {

expect(typeof "hello").toBe("string")

expect(typeof 42).toBe("number")

})

```

---

Pattern 2: Multiple Related Assertions

Documentation:

```javascript

let a = "hello"

let b = "hello"

console.log(a === b) // true

let obj1 = { x: 1 }

let obj2 = { x: 1 }

console.log(obj1 === obj2) // false

```

Test:

```javascript

// From lines XX-YY: Primitive vs object comparison

it('should compare primitives by value', () => {

let a = "hello"

let b = "hello"

expect(a === b).toBe(true)

})

it('should compare objects by reference', () => {

let obj1 = { x: 1 }

let obj2 = { x: 1 }

expect(obj1 === obj2).toBe(false)

})

```

---

Pattern 3: Function Return Values

Documentation:

```javascript

function greet(name) {

return "Hello, " + name + "!"

}

console.log(greet("Alice")) // "Hello, Alice!"

```

Test:

```javascript

// From lines XX-YY: greet function example

it('should return greeting with name', () => {

function greet(name) {

return "Hello, " + name + "!"

}

expect(greet("Alice")).toBe("Hello, Alice!")

})

```

---

Pattern 4: Error Testing

Documentation:

```javascript

// This throws an error!

const obj = null

console.log(obj.property) // TypeError: Cannot read property of null

```

Test:

```javascript

// From lines XX-YY: Accessing property of null

it('should throw TypeError when accessing property of null', () => {

const obj = null

expect(() => {

obj.property

}).toThrow(TypeError)

})

```

---

Pattern 5: Specific Error Messages

Documentation:

```javascript

function divide(a, b) {

if (b === 0) throw new Error("Cannot divide by zero")

return a / b

}

```

Test:

```javascript

// From lines XX-YY: divide function with error

it('should throw error when dividing by zero', () => {

function divide(a, b) {

if (b === 0) throw new Error("Cannot divide by zero")

return a / b

}

expect(() => divide(10, 0)).toThrow("Cannot divide by zero")

expect(divide(10, 2)).toBe(5)

})

```

---

Pattern 6: Async/Await Testing

Documentation:

```javascript

async function fetchUser(id) {

const response = await fetch(/api/users/${id})

return response.json()

}

```

Test:

```javascript

// From lines XX-YY: async fetchUser function

it('should fetch user data asynchronously', async () => {

// Mock fetch for testing

global.fetch = vi.fn(() =>

Promise.resolve({

json: () => Promise.resolve({ id: 1, name: 'Alice' })

})

)

async function fetchUser(id) {

const response = await fetch(/api/users/${id})

return response.json()

}

const user = await fetchUser(1)

expect(user).toEqual({ id: 1, name: 'Alice' })

})

```

---

Pattern 7: Promise Testing

Documentation:

```javascript

const promise = new Promise((resolve) => {

resolve("done")

})

promise.then(result => console.log(result)) // "done"

```

Test:

```javascript

// From lines XX-YY: Basic Promise resolution

it('should resolve with correct value', async () => {

const promise = new Promise((resolve) => {

resolve("done")

})

await expect(promise).resolves.toBe("done")

})

```

---

Pattern 8: Promise Rejection

Documentation:

```javascript

const promise = new Promise((resolve, reject) => {

reject(new Error("Something went wrong"))

})

```

Test:

```javascript

// From lines XX-YY: Promise rejection

it('should reject with error', async () => {

const promise = new Promise((resolve, reject) => {

reject(new Error("Something went wrong"))

})

await expect(promise).rejects.toThrow("Something went wrong")

})

```

---

Pattern 9: Floating Point Comparison

Documentation:

```javascript

console.log(0.1 + 0.2) // 0.30000000000000004

console.log(0.1 + 0.2 === 0.3) // false

```

Test:

```javascript

// From lines XX-YY: Floating point precision

it('should demonstrate floating point imprecision', () => {

expect(0.1 + 0.2).not.toBe(0.3)

expect(0.1 + 0.2).toBeCloseTo(0.3)

expect(0.1 + 0.2 === 0.3).toBe(false)

})

```

---

Pattern 10: Array Method Testing

Documentation:

```javascript

const numbers = [1, 2, 3, 4, 5]

const doubled = numbers.map(n => n * 2)

console.log(doubled) // [2, 4, 6, 8, 10]

```

Test:

```javascript

// From lines XX-YY: Array map example

it('should double all numbers in array', () => {

const numbers = [1, 2, 3, 4, 5]

const doubled = numbers.map(n => n * 2)

expect(doubled).toEqual([2, 4, 6, 8, 10])

expect(numbers).toEqual([1, 2, 3, 4, 5]) // Original unchanged

})

```

---

Pattern 11: Object Mutation Testing

Documentation:

```javascript

const obj = { a: 1 }

obj.b = 2

console.log(obj) // { a: 1, b: 2 }

```

Test:

```javascript

// From lines XX-YY: Object mutation

it('should allow adding properties to objects', () => {

const obj = { a: 1 }

obj.b = 2

expect(obj).toEqual({ a: 1, b: 2 })

})

```

---

Pattern 12: Closure Testing

Documentation:

```javascript

function counter() {

let count = 0

return function() {

count++

return count

}

}

const increment = counter()

console.log(increment()) // 1

console.log(increment()) // 2

console.log(increment()) // 3

```

Test:

```javascript

// From lines XX-YY: Closure counter example

it('should maintain state across calls via closure', () => {

function counter() {

let count = 0

return function() {

count++

return count

}

}

const increment = counter()

expect(increment()).toBe(1)

expect(increment()).toBe(2)

expect(increment()).toBe(3)

})

it('should create independent counters', () => {

function counter() {

let count = 0

return function() {

count++

return count

}

}

const counter1 = counter()

const counter2 = counter()

expect(counter1()).toBe(1)

expect(counter1()).toBe(2)

expect(counter2()).toBe(1) // Independent

})

```

---

Pattern 13: DOM Event Testing

Documentation:

```javascript

const button = document.getElementById('myButton')

button.addEventListener('click', function(event) {

console.log('Button clicked!')

console.log(event.type) // "click"

})

```

Test (in .dom.test.js file):

```javascript

/**

* @vitest-environment jsdom

*/

import { describe, it, expect, beforeEach, afterEach } from 'vitest'

describe('DOM Event Handlers', () => {

let button

beforeEach(() => {

button = document.createElement('button')

button.id = 'myButton'

document.body.appendChild(button)

})

afterEach(() => {

document.body.innerHTML = ''

})

// From lines XX-YY: Button click event

it('should fire click event handler', () => {

const output = []

button.addEventListener('click', function(event) {

output.push('Button clicked!')

output.push(event.type)

})

button.click()

expect(output).toEqual(['Button clicked!', 'click'])

})

})

```

---

Pattern 14: DOM Manipulation Testing

Documentation:

```javascript

const div = document.createElement('div')

div.textContent = 'Hello'

div.classList.add('greeting')

document.body.appendChild(div)

```

Test:

```javascript

// From lines XX-YY: Creating and appending elements

it('should create element with text and class', () => {

const div = document.createElement('div')

div.textContent = 'Hello'

div.classList.add('greeting')

document.body.appendChild(div)

const element = document.querySelector('.greeting')

expect(element).not.toBeNull()

expect(element.textContent).toBe('Hello')

expect(element.classList.contains('greeting')).toBe(true)

})

```

---

Pattern 15: Timer Testing

Documentation:

```javascript

console.log('First')

setTimeout(() => console.log('Second'), 0)

console.log('Third')

// Output: First, Third, Second

```

Test:

```javascript

// From lines XX-YY: setTimeout execution order

it('should execute setTimeout callback after synchronous code', async () => {

const output = []

output.push('First')

setTimeout(() => output.push('Second'), 0)

output.push('Third')

// Wait for setTimeout to execute

await new Promise(resolve => setTimeout(resolve, 10))

expect(output).toEqual(['First', 'Third', 'Second'])

})

```

---

Pattern 16: Strict Mode Behavior

Documentation:

```javascript

// In strict mode, this throws

"use strict"

x = 10 // ReferenceError: x is not defined

```

Test:

```javascript

// From lines XX-YY: Strict mode variable declaration

it('should throw ReferenceError in strict mode for undeclared variables', () => {

// Vitest runs in strict mode by default

expect(() => {

// Using eval to test strict mode behavior

"use strict"

eval('undeclaredVar = 10')

}).toThrow()

})

```

---

```javascript

import { describe, it, expect } from 'vitest'

describe('[Concept Name]', () => {

// ============================================================

// [FIRST SECTION NAME FROM DOCUMENTATION]

// From [concept].mdx lines XX-YY

// ============================================================

describe('[First Section]', () => {

// From lines XX-YY: [Brief description of example]

it('should [expected behavior]', () => {

// Code from documentation

expect(result).toBe(expected)

})

// From lines XX-YY: [Brief description of next example]

it('should [another expected behavior]', () => {

// Code from documentation

expect(result).toEqual(expected)

})

})

// ============================================================

// [SECOND SECTION NAME FROM DOCUMENTATION]

// From [concept].mdx lines XX-YY

// ============================================================

describe('[Second Section]', () => {

// From lines XX-YY: [Description]

it('should [behavior]', () => {

// Test

})

})

// ============================================================

// EDGE CASES AND COMMON MISTAKES

// From [concept].mdx lines XX-YY

// ============================================================

describe('Edge Cases', () => {

// From lines XX-YY: [Edge case description]

it('should handle [edge case]', () => {

// Test

})

})

describe('Common Mistakes', () => {

// From lines XX-YY: Wrong way example

it('should demonstrate the incorrect behavior', () => {

// Test showing why the "wrong" way fails

})

// From lines XX-YY: Correct way example

it('should demonstrate the correct behavior', () => {

// Test showing the right approach

})

})

})

```

---

```javascript

/**

* @vitest-environment jsdom

*/

import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'

// ============================================================

// DOM EXAMPLES FROM [CONCEPT NAME]

// From [concept].mdx lines XX-YY

// ============================================================

describe('[Concept Name] - DOM', () => {

// Shared setup

let container

beforeEach(() => {

// Create a fresh container for each test

container = document.createElement('div')

container.id = 'test-container'

document.body.appendChild(container)

})

afterEach(() => {

// Clean up after each test

document.body.innerHTML = ''

vi.restoreAllMocks()

})

// ============================================================

// [SECTION NAME]

// From lines XX-YY

// ============================================================

describe('[Section Name]', () => {

// From lines XX-YY: [Example description]

it('should [expected DOM behavior]', () => {

// Setup

const element = document.createElement('div')

container.appendChild(element)

// Action

element.textContent = 'Hello'

// Assert

expect(element.textContent).toBe('Hello')

})

})

// ============================================================

// EVENT HANDLING

// From lines XX-YY

// ============================================================

describe('Event Handling', () => {

// From lines XX-YY: Click event example

it('should handle click events', () => {

const button = document.createElement('button')

container.appendChild(button)

let clicked = false

button.addEventListener('click', () => {

clicked = true

})

button.click()

expect(clicked).toBe(true)

})

})

})

```

---

```bash

# Run all tests

npm test

# Run tests for specific concept

npm test -- tests/fundamentals/primitive-types/

# Run tests for specific file

npm test -- tests/fundamentals/primitive-types/primitive-types.test.js

# Run DOM tests only

npm test -- tests/fundamentals/primitive-types/primitive-types.dom.test.js

# Run with watch mode

npm run test:watch

# Run with coverage

npm run test:coverage

# Run with verbose output

npm test -- --reporter=verbose

```

---

Completeness

• [ ] All testable code examples have corresponding tests
• [ ] Tests organized by documentation sections
• [ ] Source line references included in comments (From lines XX-YY)
• [ ] DOM tests in separate .dom.test.js file
• [ ] Edge cases and error examples tested

Correctness

• [ ] Tests verify the actual documented behavior
• [ ] Output comments in docs match test expectations
• [ ] Async tests properly use async/await
• [ ] Error tests use correct toThrow pattern
• [ ] Floating point comparisons use toBeCloseTo
• [ ] Object comparisons use toEqual (not toBe)

Convention

• [ ] Uses explicit imports from vitest
• [ ] Follows describe/it nesting pattern
• [ ] Test names start with "should"
• [ ] Proper file naming ({concept}.test.js)
• [ ] DOM tests have jsdom environment directive

Verification

• [ ] All tests pass: npm test -- tests/{category}/{concept}/
• [ ] No skipped tests without documented reason
• [ ] No false positives (tests that pass for wrong reasons)

---

Use this template to document test coverage for a concept page.

```markdown

# Test Coverage Report: [Concept Name]

Concept Page: /docs/concepts/[slug].mdx

Test File: /tests/{category}/{concept}/{concept}.test.js

DOM Test File: /tests/{category}/{concept}/{concept}.dom.test.js (if applicable)

Date: YYYY-MM-DD

Author: [Name/Claude]

| Metric | Count |

|--------|-------|

| Total Code Examples in Doc | XX |

| Testable Examples | XX |

| Tests Written | XX |

| DOM Tests Written | XX |

| Skipped (with reason) | XX |

| Section | Line Range | Examples | Tests | Status |

|---------|------------|----------|-------|--------|

| [Section 1] | XX-YY | X | X | ✅ |

| [Section 2] | XX-YY | X | X | ✅ |

| [Section 3] | XX-YY | X | X | ⚠️ (1 skipped) |

| Line | Example Description | Reason |

|------|---------------------|--------|

| XX | ASCII diagram of call stack | Conceptual, not executable |

| YY | Browser fetch example | Requires network, mocked instead |

```bash

npm test -- tests/{category}/{concept}/

```

Result: ✅ XX passing | ❌ X failing | ⏭️ X skipped

[Any special considerations, mock requirements, or issues encountered]

```

---

Issue: Test passes but shouldn't

Problem: Test expectations don't match documentation output

Solution: Double-check the expected value matches the console.log comment exactly

```javascript

// Documentation says: console.log(result) // [1, 2, 3]

// Make sure test uses:

expect(result).toEqual([1, 2, 3]) // NOT toBe for arrays

```

Issue: Async test times out

Problem: Async test never resolves

Solution: Ensure all promises are awaited and async function is marked

```javascript

// Bad

it('should fetch data', () => {

const data = fetchData() // Missing await!

expect(data).toBeDefined()

})

// Good

it('should fetch data', async () => {

const data = await fetchData()

expect(data).toBeDefined()

})

```

Issue: DOM test fails with "document is not defined"

Problem: Missing jsdom environment

Solution: Add environment directive at top of file

```javascript

/**

* @vitest-environment jsdom

*/

```

Issue: Test isolation problems

Problem: Tests affect each other

Solution: Use beforeEach/afterEach for cleanup

```javascript

afterEach(() => {

document.body.innerHTML = ''

vi.restoreAllMocks()

})

```

---

When writing tests for a concept page:

1. Extract all code examples from the documentation
2. Categorize as testable, DOM, error, or conceptual
3. Create test file in correct location with proper naming
4. Convert each example to test using appropriate pattern
5. Reference source lines in comments for traceability
6. Run tests to verify all pass
7. Document coverage using the report template

Remember: Tests serve two purposes:

1. Verify documentation is accurate
2. Catch regressions if code examples are updated

Every testable code example in the documentation should have a corresponding test. If an example can't be tested, document why.